1. Field of the Invention
This invention relates generally to wellbore construction and more particularly to methods for forming multi-branched wellbores from one or more access wellbore. At least one access wellbore is formed substantially in non-producing subterranean formations. This invention also relates to methods of utilizing such wellbores, including utilizing the branch wellbores for storing various devices and materials and for performing certain operations in the branch wellbores. This invention further relates to an apparatus and method for transporting equipment and materials from a source location to a desired wellbore or between different wellbores.
2. Background of the Art
To obtain hydrocarbons such as oil and gas, boreholes or wellbores are drilled from one or more surface locations into hydrocarbon-bearing subterranean geological strata or formations (also referred to in the industry as the reservoirs). A large proportion of the current drilling activity involves drilling highly deviated and/or substantially horizontal wellbores extending through the reservoir. Typically, to drill a horizontal wellbore into a desired formation, the wellbore is drilled from a surface location vertically into the earth for a certain depth. At a predetermined depth, the wellbore is dog-legged into a desired direction so as to reach the desired formation, which is usually the target hydrocarbon-bearing or producing formation. The wellbore is drilled horizontally into the producing formation to a desired length. Additional dog-legged wellbores from the same vertical wellbore are also drilled in some cases. Some horizontal boreholes extend several thousand meters into the reservoirs. In most cases, however, a single horizontal wellbore, generally referred herein as the primary wellbore, main wellbore or access wellbore, is drilled to recover hydrocarbons from different locations within the reservoir. More recently, branch wellbores from the main wellbore that extend into selected areas of the producing formation or reservoir have been drilled to increase production of hydrocarbons from the reservoir and/or to maximize the total hydrocarbon recovery from the reservoir. Such a branch wellbore herein is referred to as a lateral wellbore and a plurality of such branch wellbores extending from a wellbore are referred to as multi-lateral or multi-branched wellbores.
The primary wellbore and the multi-lateral wellbores are generally drilled along predetermined wellbore paths, which are usually determined or plotted based on existing data, such as seismic data and drilling data available from previously drilled wells in the same or similar formation. Resolution of such data is relatively low. To drill such wellbores, operators typically utilize a drill string which contains a drilling device and a number of measurement-while-drilling ("MWD") devices. The drilling device is used to disintegrate the subsurface formations and the MWD devices are used for determining the properties of the formations and for determining the downhole drilling conditions. Operators utilize the information to adjust the drilling direction.
In many cases it is desirable to form a primary wellbore in a non-producing formation and then drill branch or lateral wellbores from the primary wellbore into the target formation. In such cases, it is highly desirable to place the primary wellbore along an optimum wellbore path which is at a known distance from the boundary of the target formations. Prior art typically utilizes seismic data and prior wellbore data to decide upon the path for the primary wellbore. The resolution of such data is relatively poor. Wireline tools can be run to obtain the necessary bed boundary information. Wireline systems require stopping the drilling operations for several hours and are thus not very desirable. None of the prior art systems provide in-situ determination of the location of the boundary of the target producing formations relative to the wellbore being drilled. It is, thus, desirable to determine relatively accurately the location of the boundary of the target formation relative to the primary wellbore while drilling the primary wellbore. Such information can then be utilized to adjust the drilling direction to adjust the drilling direction to form the wellbore along an optimum wellbore path.
As noted above, current drilling methods and systems do not provide in-situ methods for determining the position of the target formation bed boundary relative to a primary wellbore that is drilled in a non-producing formation along the target formation. Current directional drilling systems usually employ a drill string having a drill bit at its bottom that is rotated by a motor (commonly referred to as the "mud motor"). A plurality of sensors and MWD devices are placed in close proximity to the drill bit to measure certain drilling, borehole and formation evaluation parameters. Typically, sensors for measuring downhole temperature and pressure, azimuth and inclination measuring devices and a formation resistivity measuring device are employed to determine the drill string and borehole-related parameters. However, none of these systems allow drilling an access wellbore at a known distance from the wellbore that is determined and adjusted while the access wellbore is being drilled.
U.S. patent application Ser. No. 60/010,652, filed on Jan. 26, 1996, which is assigned to the assignee of the present application and which is incorporated herein by reference, provides a system for drilling boreholes wherein the downhole subassembly includes an acoustic MWD system in which a first set of acoustic sensors is utilized to determine the acoustic velocities of the borehole formations during drilling and a second set of acoustic sensors for determining bed boundary information based on the formation acoustic velocities measured downhole. Isolators between the transmitters and their associated receivers serve to reduce the body wave and tube wave effects. The present invention preferably utilizes the system disclosed in the U.S. patent application Ser. No. 60/010,652 to determine the location of the bed boundary of the target reservoir relative to the primary access wellbore while drilling the primary access wellbore. The drilling direction or path of the primary wellbore is adjusted based on the bed boundary information to place the primary access wellbore at optimum distance from the target formation. Since the location of the primary wellbore is relatively accurately known in relation to adjacent formations, it enables drilling branch wellbores along optimum paths into the target formation and the non-producing formations.
In the prior art primary wellbores, a number of devices are placed to facilitate production of hydrocarbons and to perform workover services. Such devices occupy space in the primary wellbore, which may be utilized for improving the overall efficiency of the wellbore system. Such primary wellbores are expensive to construct, are relatively inefficient in transporting hydrocarbons and are obstructive if major workover is required after the completion of such wellbores. It is desirable to have branch wellbores for storing various types of equipment and materials downhole, including retrievable devices which may be utilized for performing downhole operations. It is also desirable to leave the primary wellbore substantially free of any equipment and materials which may be placed outside the main wellbore and to utilize the main wellbore primarily for transporting fluids during the production of hydrocarbons. This may be accomplished by storing certain devices in the storage wellbore and by installing the fluid flow control devices entirely in the individual branch wellbores.
It is a common practice to form a seal around an area at the intersection of the primary wellbore and the branch wellbores. The seal is usually formed between the intersecting wellbores and the formation. Since the prior art branch wellbores are formed from the primary wellbores placed in the producing formations, the seals are formed entirely within such producing formations. Seals formed in the producing formations tend to be less durable because such formations typically are relatively porous and also because of the presence of depleting hydrocarbons. It is therefore desirable to form such seals entirely within the non-producing formations.
U.S. patent application Ser. No. 08/411,377, filed Mar. 27, 1996 and Ser. No. 08/469,968, filed Jun. 6, 1995, both assigned to the assignee of this application, which are incorporated herein by reference in their entirety, disclose forming branch wellbores from a primary wellbore, wherein some of the branch wellbores are drilled outside producing formations or the reservoirs for storing chemicals for treating the hydrocarbons downhole and for re-injecting water into secondary formations. Such wellbore construction solves some of the problems with the above-noted prior art wellbore. However, these methods do not provide wellbores for storing retrievable devices therein which may be utilized downhole at a later time, such as for performing completion operations, perforating, or performing workover tasks or transferring certain chemicals from such storage wellbores to another location downhole during the drilling of branch wellbores or at a later time, such as after the hydrocarbon production has started. Additionally, such wellbores do not provide for forming seals which lie outside the producing wellbores or primary wellbores which are utilized primarily for transporting fluids during the production phase.
The present invention addresses the above-noted problems associated with formation and use of multi-lateral wellbores and provides methods for forming multi-lateral wellbores from a primary wellbore which is formed substantially in a non-producing formation. The distance between the primary wellbore and the target formations is determined while drilling the primary wellbore, preferably by acoustic methods. The drilling path of the primary wellbore is altered or adjusted based on the in-situ distance measurements to place the primary wellbore along an optimum path. The lateral wellbores are drilled from the primary wellbore in the non-producing formations and producing formations. Seals are formed at the intersection of the lateral wellbores and primary wellbore entirely in the non-producing formation. Lateral wellbores are utilized for a variety of purposes, including for storing equipment and for processing and treating fluids downhole. Fluid flow control devices are placed outside the primary wellbore. The primary wellbore is utilized primarily for flowing the hydrocarbons.